Pipeline owners typically clean their pipelines by inserting xe2x80x9cpigging devicesxe2x80x9d or xe2x80x9cpigsxe2x80x9d which travel with the product flow from launching points to receiving points. In most circumstances, the pigging devices fit tight into the pipelines, and are designed to push undesirable products (water, wax build-up) ahead of the pig where the products can be removed at the receiving points. There are several types of pigs; cleaning pigs, purging pigs, dummy tools and smart tool pigs, ball pigs, foam pigs, etc. and all of these can become blocked and form a blockage. In the case of gas pipeline systems, moisture/water can lead to ice plugs that form anywhere along the pipeline.
When pigs get stuck or ice plugs form, there is typically minimal or no product flow across the blockage. If pressuring/de-pressuring or circulation of solvents cannot dislodge the blockage, then the only recourse is to locate the blockage and attempt clearing at the site. In some cases, this means cutting out the section of pipeline with the blockage inside.
If pipeline owners experience a blockage which may be caused by a stuck pig in their pipeline, they must first locate it. Blockages caused by other problems such as ice dams may also be detected by this invention. Typically, this means excavating the pipeline at the half way point, then conducting a xe2x80x9chot tapxe2x80x9d on the line to determine if there is line pressure. If pressure is observed, then this suggests that the blockage is downstream. Conversely, if little or no pressure is observed, then this indicates the blockage is upstream. This procedure has now halved the distance of pipeline where blockage is located, and the equipment is mobilized to the quarter point and the process is repeated.
Every time a hot tap is performed, it compromises the integrity of the pipeline. In essence, the owner cannot leave the hot tap in place and every hot tap requires a cut out and pipeline repair.
It is one object of the present invention, therefore, to provide an improved method of locating a blockage in a pipeline.
According to a first aspect of the invention, therefore, there is provided a method for detecting the location of a blockage in a pipeline comprising:
selecting a position on the pipeline between a supply position and a downstream receiving position where the blockage may be located;
attaching a strain gauge to the pipeline at the selected position;
applying pressure changes to fluid, that is liquid or gas, in the pipeline on one side of the blockage sufficient to cause strain changes in the pipeline on said one side of the blockage with no corresponding strain changes in the pipeline on the other side of the blockage;
detecting by the strain gauge the presence of any strain changes in the pipeline at the selected position caused by the pressure changes and determining therefrom whether the blockage is upstream or downstream of the selected position.
Normally, although not essentially, the pressure changes will be applied upstream of the selected site at the supply position. Normally, although not essentially, the blockage is of the type caused by the blocking of the movement of a pig, but the method may be used for detecting any blockage, which may be a total blockage or may be a severe restriction which does not totally block flow but which generates a sufficiently demarcated restriction so that the pressure differences are not communicated across the blockage. In such a case, the supply position and receiving position are those used for the feeding and receiving of the pig in the pipeline operation.
Preferably the strain gauge is of the type conventionally known as a xe2x80x9cbonded resistance strain gaugexe2x80x9d which comprises a foil or wafer which is intimately attached to an outside surface of the pipeline. Such a strain gauge is of a type which uses a thin metal conductive film the resistance of which changes as it is stretched. Other types of strain gauge may also be used including piezoelectric, semiconductor, carbon resistive, bonded metallic wire, and foil resistance. The first three of these could be considered xe2x80x9cnon metallicxe2x80x9d. The choice of the device is however preferably one which can respond to changes which occur over an extended period for example several minutes to several hours and is not limited to short term or vibration type changes. Piezo electric devices are less suitable because they are not responsive to such long term changes and thus respond to changes over terms of the order of seconds or less and it is impractical in many situations to cycle the pressure at periods to which piezo devices might respond.
Uni-axial gauges as well as tri-axial or rosette gauges could be used. Selection of gauge and its orientation on the pipe will be based on pipe size, wall thickness, Maximum Allowed Operating Pressure (MAOP), expected strain relative to the ability to change pipe pressure in a timely manner.
MAOP is Energy and Utilities Board regulated. A pipeline may be licensed for a certain MAOP, but its normal operating pressure could be substantially lower. As well, during testing or pressuring to remove a blockage in a licensed pipeline in Alberta, owners are allowed to exceed MAOP by 10%, but only for these purposes.
Conductive film devices of this type which are suitable for the present invention are available from Omega Canada Inc. of Toronto, Canada. This is a Foil strain gauge SG-7/350-LY11. This is a uni-axial gauge with an active grid length of 7 millimeters with a 350 ohm bridge resistance, compensated for steel application. Longer available gauges could also be used, and certainly there are ones that have been compensated for pipe materials other than steel. But different gauges can be selected pending the service (i.e. pipe material, diameter, maximum operating pressure of pipeline (MAOP) and expected cyclic pressure changes over reasonable time frames), using the same the principle described herein.
Devices of this type are available which are disposable or one time use devices and such devices can be simply bonded to the pipe by a layer of adhesive and either left in place or removed and discarded.
Preferably an exterior coating of the pipe line is removed for the intimate attachment and the coating is replaced after removal of the device.
However the device may also be used with hardware designed to be attached to the pipe and removable from the pipe where the hardware carries the strain gauge and attaches it to the pipe in a manner which communicates the strain from the pipe to the gauge itself. Such devices are available and are known as an xe2x80x9cExtensometerxe2x80x9d.
The strain gauge may have two outputs at right angles arranged longitudinally and transversely of the pipeline. If this is used, the main measurement is made in the transverse or circumferential direction where expansion of the pipe occurs due to the increased pressure and the longitudinal measurement is used to detect changes due to temperature differences and to provide a value which can be used in compensation for temperature changes in the circumferential value. In the alternative, a single direction strain gauge can be used without compensation.
The pressure impulse applied are selected so that the maximum pressure difference is obtained bearing in mind the maximum allowable pressure (MAOP) for the pipe and bearing in mind the requirement that the pressure reach the blockage within a reasonable time period. The time to reach the blockage may take several minutes or even longer depending upon the distance and the characteristics of the material within the pipe. Longer time periods are undesirable in view of the extended testing period required.
Pressurisation of the pipe can be effected by using the conventional pump if available at the location concerned. If not, conventional pressure trucks can be used. When the pressure is released, the liquid discharged can be directed to the sump if available. If not, conventional vacuum trucks can be used to remove the excess liquid.
The non-intrusive locating system disclosed herein aids pipeline owners in locating the blockages. By using very sensitive but readily available xe2x80x9cbonded resistance strain gaugesxe2x80x9d, it can determine if there are pressure changes in the pipeline, thereby indicating whether there is blockage upstream or downstream of the test site.